90 



CLIMATIC CYCLES AND TREE GROWTH 



But another situation makes such study imperative — the prediction prob- 

 lem. Without a real knowledge of the two-year scatter cycle we can only 

 use longer cycles for estimating the future. These are obtained from Hanned 

 curves, as explained, from which the two-year cycle has been extracted by 



1810 



1850 60 



a 



I 



I I 



I I 



I I I 



I I 



I 



I 



I 



I 



I 



I ■ 



I 



I I 



I I 



i 



I 

 I 



I 

 I I 



I I 



I 



I 

 I I 



i i.i.l 



DF(Utah) 

 GC (Arizona) 

 FV( Flagstaff) 



FLU •• 



FL 



LCFD „ 

 (Ft. Defiance) 



RL(Rim) 



J (Rim) 



SR 



(Tucson area) 



SC 



Summary 



10 20 30 40 1850 60 70 80 90 1900 10 20 30 



LRI 



Coast 



Arizona 



Rockies 



Fig. 37 — "Reversal" tests for short cycles. 



a. Lesser maxima in two-year reversal test; showing agreement, 

 between groups in Arizona area. 



b. Two-year test; lesser maxima summarized for three western zones. 



smoothing. After such extraction our smoothed curves show very high cor- 

 relation, 85 per cent, with similarly smoothed records of winter rainfall. 

 Hence prediction by cycles thus obtained can apply only to a similarly 

 smoothed curve and without the two-year cycle we can only try to predict 

 what amounts to the averages between successive years. Hence the lack of 



